Image fibers are widely used in the industrial field and the medical field. An image fiber has a structure in which a plurality of optical fibers are packed into a bundle and, in general, claddings of the optical fibers are fused so that a plurality of cores are formed in the cladding (see Patent Literature 4). The number of pixels (number of cores) of an image fiber is 1000 or more, for example, and not a few image fibers have 2000 to several tens of thousands of cores. The outside diameter of an image fiber is generally several mm or less. Therefore, the diameter of the core in an image fiber is on the order of μm.
A material used to form an image fiber is glass (see Patent Literature 1) or plastic (see Patent Literature 2). A plastic image fiber is more flexible than a glass image fiber. An example of a plastic optical fiber has a core made of polystyrene and a cladding made of acrylic (see Patent Literature 2), and another example of a plastic optical fiber has a core made of acrylic and a cladding made of acrylic fluoride (see Patent Literature 3).
The optical transmission loss of a plastic optical fiber does not cause a significant problem as long as the core diameter is about 1 mm. However, when the core diameter is several μm or less, the optical transmission loss of a plastic optical fiber is large, which presents a problem. Causes for such degradation of the transmission loss include a decrease in adhesion between the core and the cladding, and the interfacial structure mismatch between the core and the cladding. As one way to solve the adhesion and interfacial structure mismatch problems, a graded-index optical fiber (GI optical fiber) is known (see Patent Literatures 5 and 6).